For many forms of cancer, early detection is essential for a favorable prognosis. The cancerous growth must be detected at an early stage before the cancer is allowed to grow and spread. Such is the case for colorectal and lung cancers. As a result, techniques have been developed to examine the colon and tracheobronchial airways for the growth of precancerous and cancerous masses.
Colon cancer is the second leading cause of cancer death in the United States today. Fortunately, most colorectal carcinomas arise from preexisting adenomatous polyps, and the risk of cancer is directly related to the size of the polyp (1% in polyps less than 1 cm, 10% in polyps between 1 and 2 cm, and 30% in polyps 2 cm or greater). Scientific studies suggest that the early detection and removal of small carcinomas and precursor adenomatous polyps reduces mortality. Therefore, current strategies for colon cancer screening focus on the early discovery of polyps. The techniques used to screen for colorectal cancer include flexible sigmoidoscopy (the use of a fiberoptic scope to examine the distal half of the colon) and fecal occult blood testing (wherein hemorrhage is detected). There is some debate on the effectiveness of colorectal cancer screening, but it has been predicted that a 30-40% reduction in mortality can be achieved with proper screening using a combination of fecal occult blood testing and sigmoidoscopy.
The National Cancer Institute and the American Cancer Society recommend colorectal cancer screening for persons of average risk who are more than 50 years old using sigmoidoscopy and annual fecal occult blood tests. The fecal occult blood test is easy to perform but is plagued by many false positive and false negative results. Sigmoidoscopy suffers from the fact that it only examines the distal half of the colon (the rectum and the sigmoid colon). This is a serious shortcoming since approximately 40% of colorectal carcinomas occur proximal to the splenic flexure and are therefore undetectable using sigmoidoscopy.
Examination of the entire colon by a barium enema or conventional colonoscopy increases sensitivity for cancer detection but also increases the risks and costs. A barium enema causes patient discomfort and/or embarrassment and exposes the patient to substantial radiation. Colonoscopy, like sigmoidoscopy, does not always examine the entire colon since the cecum is not reached in approximately 15% of colonoscopies. In addition, colonoscopy requires patient sedation, places the patient at risk for bowel perforation, and is comparatively expensive. Furthermore, with the exception of fecal occult blood testing, all of these procedures meet with significant patient discomfort.
Turning now to the tracheobronchial examination, Transbronchial Needle Aspiration (TBNA) is a bronchoscopy technique that permits the outpatient diagnosis and staging of mediastinal disease. This procedure allows for the outpatient sampling of tissue specimens that might otherwise require a surgical procedure. With TBNA, a needle is placed through an airway wall in the vicinity of a suspected lesion to retrieve a tissue sample. Conventionally, the bronchoscopist is guided only by a mental model of the patient""s anatomy and pathology following review of bronchoscopy images and/or a series of thoracic computed tomography (CT) images. As can be expected, proper placement of the needle can be extremely difficult and to a small degree somewhat imprecise.
Accordingly, it is highly desirable to have a reliable, efficient method for examining the tracheobronchial tree and/or the colon of a patient to detect early cancer. The technique should allow for the discovery of polyps of 1 cm or greater in size in the colon and 5 mm or greater in the airways. Preferably, the method should reduce the amount of discomfort encountered by the patient, decrease the risk of injury to the patient, and be conducted in a reasonable amount of time without being prohibitively expensive. Preferably, the method should be non-invasive or minimally invasive.
In accordance with the present invention, a system and method are provided for producing two-dimensional images of a selected structure, such as a portion of the body, to enable the creation of a three-dimensional rendering of the selected structure. More specifically, two-dimensional images of a selected body portion are acquired with use of a scanner, for example, a helical computed tomography (CT) scanner. The two-dimensional images are then stacked to create a three-dimensional image volume. From the three-dimensional volume, image features of one or more selected body organs are isolated or segmented. Isosurfaces of the segmented organs are produced and wireframe models are then generated from each of the isosurfaces for the respective segmented organs. The wireframe models are used to generate real time, three-dimensional images (renderings) of the selected organs.
In a specific application for generating a three-dimensional rendering of a patient""s colon, the patient initially undergoes a selected preparation procedure. For example, the patient""s colon is initially cleansed and then inflated with air to permit the acquisition of unobstructed two-dimensional images of the colon. Next, the patient undergoes a CT scan to produce a series of two-dimensional images of the patient""s internal organs. Preferably, a spiral or helical CT scanner is employed to provide a series of uninterrupted two-dimensional images through the body. The series of two-dimensional images are transferred from the scanner to a graphics computer to effect various image processing procedures. The dataset corresponding to the series of two-dimensional images may be transferred to the graphics computer in a compressed format for decompression on the graphics computer. Alternatively, the dataset representing the series of two-dimensional images may be decompressed on the computer console of the scanner prior to transfer to the graphics computer.
After transfer to the graphics computer, the series of two-dimensional images are stacked in order to form a three-dimensional volume file. In order to facilitate three-dimensional rendering of a selected organ contained within the three-dimensional volume of images, the three-dimensional volume file may be subjected to various optional dataset reduction techniques. For example, a reduction of pixel resolution on the series of two-dimensional images may be effected. In addition, the three-dimensional volume file may be separated into selected subvolumes.
After the optional dataset reduction procedure is completed, an image segmentation process is performed in order to isolate features of a selected organ or region of interest from the three-dimensional volume file. Image segmentation may be effected by various techniques. For example, an image slice through the three-dimensional volume file may be subjected to a thresholding process in which a physical property of the two-dimensional image slice, such as x-ray attenuation, may be used to establish a particular threshold range, such as a range of x-ray attenuation values, that corresponds to the organ of interest. After an appropriate threshold range is determined, the entire three-dimensional volume file is then thresholded to segment the organ of interest. For example, in order to segment the colon, a threshold range corresponding to the air column within the colon could be selected to isolate the inner wall of the colon.
An alternative segmentation technique may be employed in which a region growing technique is used to isolate the air column within the colon. Using the region growing technique, a xe2x80x9cseedxe2x80x9d is planted by selecting a data point or voxel within the air column of the colon. Neighboring voxels are progressively tested for compliance with a selected acceptance criteria, such as x-ray attenuation values falling within a selected threshold range representing air. As such, the seed region continues to expand or grow until the entire air column within the lumen of the colon is filled.
A surface, or isosurface, of the air column representing the colon is then produced. A wireframe model of the isosurface is then generated using a selected image processing technique such as a marching cubes algorithm. From the wireframe model of the colon, a three-dimensional interactive rendering is produced that enables the user to rapidly view a series of three-dimensional images of the lumen of the colon for purpose of detection of pathological conditions. As such, a user has the perception of flying through the lumen and virtual reality environment is achieved.
Other procedures may be employed with the interactive, three-dimensional rendering of the organ for purposes of medical diagnosis or observation. For example, a three-dimensional rendering of the colon may be split open so that the interior half of the colon along a selected length may be viewed. In addition, selected flights or paths of movement through the lumen of the colon may be recorded on videotape or, alternatively, stored in the memory of the graphics computer as a path of coordinates for subsequent reproduction and display. In addition, various areas of pathology may be marked by different colors during a flight through the lumen of the colon to facilitate subsequent detection.
The method and system of the present invention may also be used to isolate other organs. For example, the system and method may be used to isolate the tracheobronchial airways, as well as the surrounding lymph nodes and blood vessels. During an interactive, three-dimensional rendering, a selected organ may be rendered transparent to facilitate or permit a view of the remaining (surrounding/neighboring) organs. For example, the airway walls may be made transparent so that the surrounding blood vessels and lymph nodes become visible from a perspective within the interior of the airways.